The present invention relates in general terms to high voltage transmission lines and cables.
It is more particularly directed at reducing the breakdown risks inherent in all solid dielectric insulants used in the aforementioned transmission means, said risks increasing during the aging of the insulant.
Transmission cables and lines have a great diversity as a function of the voltage, current and frequency ranges under which they are used. The transportation of high power levels up to a place of use often takes place with lines having 2, 3 or 4 conductors or wires under a low frequency, e.g. 400, 60 or 50 Hz and in direct current form.
Signals are generally transmitted by a twin-conductor line or by a coaxial line as a function of the frequencies to be transmitted and the transmission conditions.
For reasons of clarity and simplification, a description will only be given here of coaxial cables, but it is obvious that the invention applies to all insulants used in the different types of lines, the line portions constituting insulant "passages" and the high voltage means having solid insulants.
One of the main reasons for high voltage equipment failing is an electric breakdown, which occurs between the conductors across the insulants or insulators (sometimes preceded by a partial electric discharge) leading to the perforation of the insulant and the latter may even be destroyed. The failure occurs in a random manner, but it is found that this risk increases greatly over a period of time as a result of the ageing process.
This problem of the reliability and service life of insulants is a considerable preoccupation for the manufacturers of cables and this state has existed for a long period of time. The hitherto very limited knowledge of the physical phenomena involved has not made it possible to find effective solutions.
Since, by definition, lines have a very considerable length compared with the wavelength, the approximation of standing waves cannot be applied thereto. The study of the transmission, energy absorption and therefore ageing phenomena can only take place on the basis of MAXWELL equations, conditions at the limits defined by the separating surfaces between the different media and relations specific to the media in question.
Without wishing to go into detail concerning the theory of transmission lines, it is pointed out that for a perfect conductor, the local charge density is zero, whereas for a dielectric the local electric field can be considered as the sum of the electrostatic field due to the electric charges developing therein and the field calculated by the MAXWELL equations in the uncharged medium.
If the media were perfect, the electric and magnetic fields would be zero in the conductors end would be in phase and unattenuated in the insulants, in such a way that the electric and magnetic energies would be equal. In practice, conductors and insulants are not perfect and two types of faults or defects occur, namely volume defects and interface defects.
The standard process for increasing the breakdown threshold of a coaxial cable consists of increasing the dielectric material thickness. However, a limitation is obviously imposed by dimensional considerations (size and price) and in any case the aging faults remain. Other ideas and various processes have been conceived, all based on the fact that the existence of charges in the insulating dielectric mass is inevitable and that their prejudicial effects are limited by the application of other fields aimed at deflecting them, or by reducing the fields as a result of special geometrical arrangements.
For example, FR-A-2,349,932 (Ser No. 7703498) describes a transmission line having a device located outside the dielectric region and whose aim is to create a magnetic field, so that the charged particles present in the dielectric undergo a helical movement. Thus, the probability of a charged particle reaching the energy level corresponding to ionization becomes very low. This process is without doubt effective in the case of a cable having a gaseous insulant (e.g. SF.sub.6), but it is not justified for a solid insulant, which is the general case for industrial applications.
Using different procedures, attempts have already been made to improve the operation of the dielectrics of electric cables and give them better performance characteristics with respect to the constraints and stresses which they undergo.
Reference is e.g. made to FR-A-2 357 992, which describes an electric cable in which the insulant has a gradation of the permittivity from the central conductor to the external conductor. This gradation is obtained by using insulating layers in which large quantities of oxides (80 to 100%) have been added. The objective of this document is to minimize the prejudicial effects on the breakdown of a lack of symmetry or concentricity of the cable. In addition, CH-A-669 277 describes a cable having several layers of insulating material with different permittivities. The aim is to improve the manufacturing constraints by reducing the thickness of the insulator.
However, neither of these documents discloses means for specifically combatting the increase in breakdown risks as a function of aging.